In normal cells, the decision to proliferate (to rapidly divide and grow) is tightly regulated by a sophisticated network of intracellular signaling proteins. Improving our understanding of this and other cell signaling processes is important because many complex diseases result from defects in this intracellular decision-making machinery. For instance, cancer can be thought of as a disease borne out of abnormalities in the growth control network that lead to uncontrolled cell proliferation. In this proposal, I will focus on the signal processing mechanisms that determine one critical decision: the activation of T cells within the adaptive immune system. T cells make an ideal test bed for studying cellular information processing, since many of the isolated signaling components have been identified and because T lymphocytes show particularly exquisite control of proliferation. In fact, while T cells are normally quiescent and robust to environmental noise, they can be stimulated by minute quantities of pathogenic ligands (fewer than ten molecules) and then proliferate at rates rivaled only by embryogenesis. Intracellular signal processing mechanisms invoking signal dynamics and combinatorial control of input signals could explain this remarkable phenomenon, but we lack a predictive and mechanistic understanding of the properties of the T cell signaling network that lead to these signal processing behaviors. In large part this deficit in our understanding is due to the fact that we lack the quantitative tools to probe the inside of a living T cell and rigorously interrogate which nodes are critical for dynamic and combinatorial T cell signal processing behaviors, and which are not. I propose to use recently developed optogenetic protein switches (light---gated protein---protein interaction modules) that can be used as knobs to selectively activate key intracellular signaling nodes with passive light illumination. Using optogenetic tools I can for th first time systematically vary the duration and combination of input signals at sequential nodes in the T cell activation network, and quantitatively measure changes in signaling output and functional T cell behavior. The systematic comparison of T cell behavior at different points in the T cell activation network will reveal how dynamic and combinatorial signal processing mechanisms within T cells lead to highly regulated control of T cell activation and amplified growth. This work should provide fundamental insight into how biochemical networks sense and process information, and the network and physical properties that enable useful behaviors such as noise filtering and amplification. In the long---term, these studies could influence the design of novel combinatorial biologic therapeutics or T cell---based immunotherapies for chronic diseases such as cancer or autoimmune disorders.